1. Field of the Invention
The present invention relates to a receiver, more particularly relates to a receiver for receiving a digital signal such as a radio wave modulated by the orthogonal frequency division multiplexing (OFDM) scheme.
2. Description of the Related Art
The OFDM scheme is used for ground wave digital television broadcasting. The OFDM scheme is a type of a multi-carrier transmission system and transmits a signal obtained by quadrature amplitude modulation (QAM) of 5300 sub-carriers arranged at intervals of for example 1 kHz.
The multi-carrier system is known as a system generally excellent in durability with respect to frequency selective phasing. Frequency selective phasing causes a problem in that it degrades the line quality in broadband wireless communication. In the ground wave digital television broadcasting, it is necessary to transmit high quality images, so the OFDM scheme is employed in the about 500 MHz to 800 MHz frequency band.
In the OFDM scheme, the frequency bands of the modulated waves are superimposed on each other between adjacent sub-carriers, but the orthogonality of the modulated wave band signals, that is, the zero correlation, is utilized for block modulation/demodulation using a Fast Fourier Transform (FFT). Further, by adding a guard interval signal on the transmission side, inter-symbol interference (ISI) due to the multipath delay is eliminated.
FIG. 1 shows an example of the configuration of a conventional OFDM receiver mounted in a vehicle. Further, FIG. 2 shows an example of a procedure for switching between the directional antennas of FIG. 1. In FIG. 1, an OFDM receiver 11 receives a radio wave modulated by the OFDM scheme. The antenna unit is configured by a front beam antenna 12A having a directivity giving a high gain in the front of the vehicle, a rear beam antenna 12B having a directivity giving a high gain in the rear direction opposite to that, and a switch 13 for selecting one of these directivities.
A demodulation unit is configured by an RF/IF unit 14, an OFDM demodulation unit 15, a level detection unit 16, an error correction unit 17, and an AGC unit 18. Here, the RF/IF unit 14 amplifies the received RF signal, converts the result to the intermediate frequency, and further amplifies the same. The OFDM demodulation unit 15 for demodulating the signal of the intermediate frequency includes the level detection unit 16 and the error correction unit 17 and demodulates and outputs the valid symbols (TS). The level detection unit 16 outputs the reception power information and adjusts the gain of the RF/IF unit 14 via the AGC unit 18.
The error correction unit 17 utilizes the error correction information included in the demodulated signal to detect error and correct it to the possible extent and outputs information of the error rate (BER: Bit Error Rate). The antenna selection circuit 19 controls the switch 13 to select the suitable directional antenna 12A or 12B based on the reception power information and error rate information.
FIG. 2 shows an example of the procedure for switching between the directional antennas in a antenna selection circuit 19 of the OFDM receiver 10 of FIG. 1. At step S11, the circuit monitors if the reception error rate becomes larger than a predetermined reference value. Here, the fact of the reception error rate becoming larger than the predetermined reference value is made the condition for starting antenna switching control for the switch 13.
When the reception error rate is smaller than the predetermined reference value, error correction is possible or the effect of the error is small in range, therefore it is not necessary to switch the directivity of the antenna unit. If the reception error rate becomes larger than the predetermined reference value (Yes of S11), first the front beam antenna 12A side is switched to for a predetermined period (several hundreds of milliseconds) and the mean reception level thereof is found (S12).
Next, the rear beam antenna 12B side is switched to for a predetermined period (several hundreds of milliseconds) and the mean reception level thereof is found (S13). Then, these mean reception levels are compared (S14), then the antenna of the side where the reception level is higher is selected and that state is maintained for a constant time (several seconds to several tens of seconds) (S15). After the elapse of the constant time, the above operation is repeated (S11 to 15).
In this way, conventionally, the practice had been to successively switch to a plurality of antennas having different directivities for predetermined periods to detect their mean reception levels and compare them to select the suitable directional antenna. Due to this, the effect due to multipath phasing and Doppler shift when a moving vehicle received a digital television broadcast signal was reduced (see FIGS. 1 and 2 of Japanese Unexamined Patent Publication (Kokai) No. 2003-283405).
Especially, due to Doppler shift, when a high speed moving vehicle receives a digital television signal modulated by the OFDM scheme, if there is frequency selectivity in the multipath, the reception property will be degraded due to inter-carrier interference (ICI). For this reason, the practice has been to mount a plurality of directional antennas having different directivities on the vehicle and switch the antenna directivity so as to reduce the effect of the inter-carrier interference in a multipath environment (see Japanese Unexamined Patent Publication (Kokai) No. 2003-087213).
Note that, separate from the configuration of FIG. 1, even when providing the OFDM receiver with two systems of antenna means and OFDM demodulating means for reception by the frequency division diversity scheme, the same approach using a switching procedure as in FIG. 2 can be applied (see Japanese Unexamined Patent Publication (Kokai) No. 2003-283405).
If, however, like in above conventional examples, employing the routine of successively switching to a plurality of directional antennas for predetermined periods to detect mean reception levels and comparing these to select the antenna having the best orientation, when for example a vehicle moving at a high speed on streets surrounded by high-rise buildings switches to the desired directional antenna based on the results of the comparison, there would be the problems that the reception environment of the radio wave would already be changing and therefore the state of reception of the vehicle might not be improved or might even be degraded.
Especially, when receiving a digital television signal, there has been the problem that not only does reception deteriorate like with an analog television signal, but also loss of synchronization etc. can lead to reception suddenly being disabled. Further, according to the example of FIG. 2 (S15), there has been the problem that this poor state of reception has to be endured for a certain time (several seconds to several tens of seconds).
Further, with a reception antenna switching scheme, like in the above conventional examples, of successively switching to a plurality of directional antennas for predetermined periods to detect the mean reception levels and comparing them to select the antenna having the best orientation, sometimes a directional antenna having an excessive reception level is selected. As a result, there has been the problem that reception was degraded or reception was disabled.
FIG. 3 shows an example of the settings for switching directional antennas in the prior art. In this example, a broadcast radio wave of a digital television broadcast signal (OFDM signal) is transmitted from a radio tower 201. Further, a vehicle 202 is provided with a front beam antenna 203 facing the direction of advance and a rear beam antenna 204 facing the opposite direction. In this example, the vehicle 202 has entered the strong field of the radio wave transmitted from the radio tower 201. According to the conventional method of switching of directional antennas explained above, the front beam antenna 203 facing the direction of arrival of the radio wave is selected.
In this case, the combination of the strong field of the radio wave and the directional antenna 203 facing the direction of arrival of that radio wave results in the reception level of the radio wave becoming excessive. There was therefore the problem that this excessive reception level exceeded the permissible operating ranges of the amplifier, attenuator, etc. of the reception tuner mounted in the vehicle 202 and caused waveform distortion, harmonic noise, etc., and rather caused deterioration of the reception property of the digital television broadcast signal.
Further, if just, like in the above conventional example, successively switching to a plurality of directional antennas for predetermined periods to detect their mean reception levels and comparing them to simply switch to the antenna having the best orientation, there has been the problem that the reception level greatly fluctuates before and after switching the antenna.
FIG. 4A and FIG. 4B show an example of the configuration of a conventional switch 13. FIG. 4A shows an example of the circuit configuration of the switch 13, while FIG. 4B shows an example of the operation timing thereof. In FIG. 4A, one of the reception signals from the front beam antenna 12A (antenna A) and the rear beam antenna 12B (antenna B) is input to the RF/IF unit 14 via a switch 351 or 352 selected by a control circuit 354. As shown in FIG. 4B, the switches 351 and 352 are simultaneously switched by the control unit 354. In this example, the antenna A is switched from on (connection) to off (disconnection), while the antenna B is switched from off (disconnection) to on (connection) simultaneously.
In such a conventional configuration, the AGC circuit 18 can not track the fluctuations in level before and after the switching, so an impulse noise is generated due to the filter transition response of the later RF/IF unit 14. Further, there has been the problem that a delay arose until the reception level was stabilized, therefore a sharp change of level of the reception signal occurred and exceeded the permissible difference of sampling of the next stage A/D converter etc. causing deterioration of the state of reception and consequently causing bit error.